Casting my mind back to Plate tectonics classes 20 years ago, energy release to achieve equilibration is what earthquakes are all about. The audible bang is only one of the several ways in which an earthquake releases energy. The others are heat and movement. If the sound did indeed precede the shake, it'd prove that the energy released travels faster through the air (in the form of sound waves) than through the crust (as motion waves). I forget which is which.

As the North Island sits right atop two plates which are trying to go in opposite directions it is never going to be a stable place to live. I recall my lecturer in 1977 predicting that the North Island would cease to exist within a century - apparently the two plates have been stuck together for way too long to be healthy.

As regards your question - hows about "backslide". (Must admit I didn't have to think too much about that one - thanks for the leg up!! Will think further on the matter no that the geology lesson has been delivered.

Casting my mind back to Plate tectonics classes 20 years ago, energy release to achieve equilibration is what earthquakes are all about. The audible bang is only one of the several ways in which an earthquake releases energy. The others are heat and movement. If the sound did indeed precede the shake, it'd prove that the energy released travels faster through the air (in the form of sound waves) than through the crust (as motion waves). I forget which is which.

As the North Island sits right atop two plates which are trying to go in opposite directions it is never going to be a stable place to live. I recall my lecturer in 1977 predicting that the North Island would cease to exist within a century - apparently the two plates have been stuck together for way too long to be healthy.

As regards your question - hows about "backslide". (Must admit I didn't have to think too much about that one - thanks for the leg up!!) Will think further on the matter now that the geology lesson has been delivered.

As one who has lived in that bowl of jello called Mexico City, I know what you mean about the jiggles. I was never there during anything strong, but I can imagine it as being unsettling. My sympathies. How about motorhome?

Soundbite, waterfall, powercut are just 3 examples in which, like in your two, the second part can be read as a noun as well as a verb. Stretching the point only a little further, cornerstone, woodfire and cardhouse would also qualify.

Per Stales' and Consuelo's comments, the entire Pacific is known as The Ring of Fire to geologists and vulcanologists, since it seems that the volcanoes that one finds on both sides of the Pacific (now THERE'S a misnomer!) indicate where the fractures in the plates are. We here in Oregon had a shake of equivalent magnitude (but no magma-tude!) last year, and, of course there was Mt Saint Helens in 1980. I heard quite a bang from that, and I then lived over 200 Km away.

As for noun/verbs, probate is a bit of a stretch, but it's too early here to think.

Hi Bill,Let me have a try at explaining: The "liquid" magma below the Earth's crust is to be compared to porridge in a pot on a hotplate. Just the scale (space - viscosity -> time) is different. Due to the thermal gradients (complicated by chemical reactions and magnetic forces), it rises at certain places, thus in other places, it has to sink. The plates of the crust are like dried flakes of the porridge. They cannot follow the downward movement of the highly viscous liquid, and are thus squeezed together.

Dear wsieber: The thing that amazes me is that the movement of the plates is so steady for so many years. The "hot spot" that produced the Hawaian Islands seems to have had a fairly constant change of position.Convection in astoundingly slow rate. What drives it?

The magma on which the continental plates "float" has currents which appear to remain relatively stable in terms of direction and velocity for long periods of time. A bit like the Gulf Stream, I guess. The force of these currents is great enough to force the plates together for what appears to us to an amazing amount of time. The Pacific plate subducts under the Australian plate, from memory and New Zealand kind of trembles on the brink, so to speak. Incidentally, the Himalayas and the Tibetan plateau are all the result of the plate on which India sits pushing up at the southerly end of the Asian plate. Some force, huh?

It's driven by magma being forced up between the plates in the centre of the major oceans - the Pacific and the Atlantic. The mid-Atlantic ridge is highly volcanic. New plate material is formed by the magma and is pushed away from the scene of its creation. Or something like that, anyway.

It's all been proved by the study of changes in magnetic polarity in the material on the seabed. Turns out that the patterns of magnetic change are mirror images of each other on each side of the mid-Atlantic ridge, showing that the material on both sides was created at the same time.

Dear Max,That "Bang" you heard at the time of the earthquake might have been the audible manifestation of a phenomenon that has been many times reported but never photographed - a flash of electromagnetic light caused by the sudden release of energy by the earthquake. A direct transfer of energy, as Stales suggests- from medium (rock) to medium (air), would seem likely to produce a rumble more so than a bang. A thunderclap produces a similar "Bang" as air rushes in to fill the vacuum left by a lightning strike.

Hi Milum,A thunderclap produces a similar "Bang" as air rushes in to fill the vacuum left by a lightning strike.. Unfortunately this is an old erroneous myth. The lightning strike does not leave a vacuum. It simply heats up the air incredibly quickly, and the resulting expansion makes itself felt as a bang. The crackling sparks of static created when you pull off your acrylic sweater, are miniature models of the phenomenon.

aren't new erroneous myths called urban legends? and all are identified by having happened to "a friend of a someone i work with cousin" ? (there being six degrees of separation between strangers in the modern world but only three degrees of separation in an legend.. this being one of its major characteristics.)

Ah - you take a few days off to go bush and everyone horns in on your special subject. Never mind - thanks for all the learned input.

Must however correct one slight inaccuracy:

"It's driven by magma being forced up between the plates in the centre of the major oceans - the Pacific and the Atlantic. The mid-Atlantic ridge is highly volcanic. New plate material is formed by the magma and is pushed away from the scene of its creation. Or something like that, anyway."

The subduction of one plate under another isn't really "driven" by the magma at the opposite side of the plate forcing its way up. As mentioned in an excellently descriptive post, the convection currents within the mantle (driven, it is postulated, by the heat generated from the radioactive decay of the minerals present) carry the crustal plates across the surface of the earth - rather like a conveyor belt.

Where they collide, the crust on one side of the impact zone buckles and heaves - building mountain chains like the Himalayas and the Andes. The other plate is "subducted" (ie pushed under the other) - the line along which this happens often forming a deep ravine ("trench") in the ocean (eg the Mariana Trench off the east coast of Japan). The crustal material that melts has a lower density than the surrounding mantle and rises slowly through it, eventually cooling at or near the surface as granitic or related rocks on the far side of the mountain range.

Crustal material delivered into the subduction zone at one side of the plate must be replaced - this happens at the Mid Ocean Ridges (MOR's). The magma that comes to the surface at the MOR's is filling the void left by the plates as they move apart along the line of the MOR. We've all seen pictures of this happening - especially the Mid Atlantic Ridge where it comes to the surface in Iceland.

Not all plates meet head on. The movement between the Pacific plate (upon which half of NZ's North Island sits) and the Australian plate is lateral and, as mentioned in the previous post, they've been locked together for a long time. Simply put, the rate of movement at the join isn't keeping up with the movement away from there. It has to release sooner or later - and all Kiwis should be hoping for a lot of smaller quakes, rather than one big mutha.

I've seen time projection maps that say that because all of the continents are moving they will again form one big continent where the Pacific ocean is now. Could it be possible, though, that while the plates are moving they keep pushing the Mid Atlanic Ridge up and it eventually rises into a new continent?

Jazzo, anything is possible given enough time. the east coast mountains, the Appalachians, now eroded, and derided as meer hills, used to be as high as the himalayas! almost everything south of the hills of arkansas is mississippi delta-- about 1000 miles! (the result of the erosion of the same mountains into hills! )

some of the rocks found in Marin county, north of Frisco, are from area near LA. I like geology, i have trouble comprehending the vast number of years, (eons!) it has taken to create, say the 1000 mile mississippi delta, but a good geological map, gives lots of information. I'll look for a link on Monday and send it to you as PM-- the US geological service has some great ones. I don't know of any really great world ones, but each country has good ones of there own-- maybe stales can get one of down under.. and so on..and for further information, i would recommend The Map that Changed the World, by simon winchester, the author of The Professor and the Madman. a short history of the first geological map, and other information about the beginnings of the science of geology.

PS. you should know some geology-NYC's skyline is shaped by the underling geology of manhattan island... in NY case,geology is destiny! after all the Bronx is gneiss, Manhattan is schist*, and NY is not with out its faults!*bronx natives have at tendency to mispronounce this word rudely.

Mid ocean ridges are predominantly places of tension, resulting in cracking of the Earth's crust. There is no pushing of the adjacent plate - in fact they move apart at a combined speed of up to a metre per year.

The ridge itself is a product of volcanic activity. Just like any volcano, there's a build up of solidified lava around the mouth. The upwelling magma also pushes the ridge up - rather like inflating a balloon under a blanket.

Dear stales: I have read that much of the heat in core of Earth is from decay of radioactive elements. Can the hot spots be due to fact that the radio-active material arrived in large lumps that have not become homogenized in four billion years?

Although far from an expert on the matter, I don't believe hot spots are localised pods of higher radioactive decay. Let's play visual games...

Hold your hands up level, palms down, in front of you to represent two plates. Bring your fingertips together (convergenet crustal movement), then slide one hand under the other (subduction). The leading edge of the plate that's been subducted (fingertips of your lower hand) starts to melt - note this occurs below and to one side of the impact zone. Because it's hotter (simply through being closer to the fire in effect), the newly melted part is less dense than the surounding upper mantle and crust. Being less dense, it rises towards the earth's surface (ie up towards the middle of the palm of your top hand). Once it hits the earth's surface you've got a hot spot.

The upwelling may never make it to the surface if (frinstance) it is not significantly hotter than the surrounding material or if it cools before it gets there.

A lava lamp is an excellent example of how it all works. The waxy stuff is denser than the surrounding liquid when cool so initially sits at the bottom of the flask. Once heated (and thus less dense), gobs of the stuff rise to the surface where they cool and sink again.

When I last studied the subject, it was thought that the molten part of the interior earth is a function of radioactive decay and gravity. It's a hangover from the earth's molten beginnings. The fact that we're still hot and some other planets aren't is (I think) a product of the earth's size (and thus surface area). Surface area relative to volume increases in inverse proportion when talking of spheres - that's why babies lose so much heat through their heads. Thus the smaller planets have all undergone a greater rate of cooling and the bigger ones are still boiling clouds of gas.

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